This invention relates to treatment of carbon fibers and, in particular, to the treatment of the surface of carbon fibers in order to increase the active surface area, total surface area, and surface roughness of these fibers.
Carbon fibers because of their unique combination of properties are finding increased use in fields as diverse as energy, sporting goods and aerospace. Because of their relative chemical inertness, they are finding use as a catalyst support in fuel cells and in numerous other chemical reactions. In a structural composite the fiber properties that are most useful are their high strength, high modulus, and low density. At elevated temperatures these fibers become even more attractive in structural composites because they have very significant strength and modulus up to 3000.degree. C. Thus, it is the matrix material and not the fiber that determines the composites useful temperature envelope. For applications up to about 300.degree. C. the matrix is usually an epoxy or a phenolic, while at higher temperatures a metal matrix can be used. At temperatures above 1200.degree. C. the matrix must be a ceramic or carbon itself. These carbon-carbon composites are very useful materials that have found wide application because they are stronger and stiffer than steel, have a lower density, and maintain their properties to very high temperatures.
Carbon fiber composites can be tailored to have a wide range of properties. Apart from using different carbon fibers this can be accomplished by modifying (1) the fiber architecture, (2) the matrix material, or (3) the degree of fiber-matrix bonding. It is possible to modify the fiber-matrix interface by changing the fiber's surface roughness, or the degree of chemical interaction between the fiber and the matrix.
The degree of chemical interaction between the fiber and the matrix, which is the most important of these three parameters, can be enhanced in order to increase the tensile strength of the composites. This results in a decreased failure rate due to fiber pull-out under tensile stress.
This enhancement in chemical bonding can be accomplished by increasing the fiber's active surface area (ASA) which is composed of all the sites on a carbon fiber surface capable of forming a chemical bond. These sites are located on the carbon surface wherever the valence is not satisfied. Typically, the majority of these sites are located at the edges of the basal planes but active sites are also located at any imperfection in the basal plane such as vacancies, dislocations, interstitials, etc.
It is the ASA that also acts as bonding sites for metal particles placed on the carbon fiber surface to serve as catalysts. Carbon fibers as a catalyst support find application in numerous chemical reactions such as in fuel cells, heterogeneous reactions, and as electrodes in electrochemical processes. Carbon fibers in these applications improve mechanical properties and give better thermal shock resistance. For this reason, it is desirable to significantly enhance the size and number of ASA patches on a carbon fiber surface used as a catalyst support. This ASA enhancement would increase both the amount of catalyst that could be placed on the surface and its degree of dispersion. Both of these parameters have a significant effect on the efficiency of the supported catalyst.
Further, the carbon active sites also serve as nucleation sites for any deposit or coating. In many cases, such as for oxidation protection, it is desirable to coat carbon fibers or composites made from them. To accomplish this it is necessary to have a significant number of ASA patches with a certain minimum size in order to bond coatings, which are composed of molecules much larger than an oxygen molecule, to the fiber surface.
Traditional manufacturing process to increase the number of carbon fiber active sites include oxidation in air, nitric acid, or an electrochemical cell. The limitation of all these techniques is that they only increase the size of ASA patches already present on the surface but are unable to create ASA patches in the perfect basal plane areas.
On the other hand, alternate techniques such as plasma etching in argon or oxidation in atomic oxygen, in addition to removing edge sites are able to remove basal plane atoms and create ASA patches where none existed previously. However, even these process are not as effective in increasing the fiber active surface area as catalytic oxidation. Some of these prior process are disclosed in the following U.S. Patents which are incorporated by reference:
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